In soldering electronic components, circuits, equipment and the like, various kinds of fluxes are used together with the solder so as to improve the efficiency of the soldering operation, to secure the soldered connections and to improve the long-term reliability of the connections while maintaining proper electrical performance. Conventionally, fluxes may be divided into three broad categories: (1) natural rosin; (2) activated (mildly or fully activated) rosin having a halogenated compound or organic acid activator incorporated with natural rosin; and (3) rosin free flux, generally referred to as water soluble or synthetic flux.
Natural rosin flux is a stable flux but results in a solid rosin flux residue which, if not completely removed, results in serious contact resistance problems in electronic relays, connectors, goldfingers and printed wiring board circuitry. Excessive rosin flux residues also prevent the adherence of protective coatings commonly applied to finished electronic circuit assemblies. This type flux is advantageous in that it presents few problems with respect to corrosiveness.
Activated rosin flux has a stability similar to natural rosin flux and causes little corrosion at room temperature. Fully activated fluxes have a strong fluxing action at soldering temperatures due to the activators added thereto. Such activators, e.g., amine hydrochlorides, are typically present in high concentrations such as 1 to 10 weight percent of the resultant flux. However, a fully activated rosin flux has disadvantages in that at soldering temperatures a corrosive gas is produced. Moreover, the residues of the activated rosin combine with moisture to produce corrosive acid. Presently available fluxes containing organic amine hydrohalides in the form of neutral salts such as glutamic acid hydrochloride, either form corrosive metal halides at elevated temperature or the residues thereof combine with moisture at room temperature to form a corrosive acid and thus are used with possible deleterious effects for electronic soldering applications.
There are also mildly activated rosin fluxes such as those taught in U.S. Pat. No. 4,168,996. While these are not corrosive, they still suffer from the same disadvantage of rosin flux in that the flux residue is difficult to remove.
Rosen free fluxes are very efficient in removing oxides from the metal surface to be soldered. They generally have the distinct advantage of being relatively easily removed from the device by washing with a suitable solvent, alkaline detergent or water. However, there are disadvantages to their use because they frequently contain either strong acids, such as hydrochloric acid or strong organic acids, or an inorganic salt which which hydrolizes in water to give an acid reaction. Therefore, they are apt to destroy metallic material or to leave residues which corrode the soldered parts after soldering, thereby resulting in decreased reliability of the soldered parts with respect to electrical and mechanical properties.
Furthermore, some rosin free fluxes comprise polyethylene glycol and/or its derivatives as a flux vehicle. It has been found that such fluxes interact with the polymeric surfaces of electronic devices, e.g. printed wiring boards, to modify them so that they become more conductive, thereby increasing still further the chances of device malfunctioning. The change in substrate surface quality with regard to conduction is measurable as a decrease in insulation resistance. For most soldering techniques, the above mentioned disadvantages have been substantially alleviated by a rosin free, water soluble flux as set forth in U.S. Pat. No. 4,342,607 when employing low or medium temperature soldering operations, e.g. 350 degrees to 500 degrees F. However, when the soldering operation is a higher temperature operation or for extended periods of time, or when greater flux activation is necessary, such as in infra red solder reflow or hot gas leveling techniques, greater thermal stability and/or flux activation is often required. Such high thermal stress exerted on the printed wiring board material makes the soldering process particularly sensitive to the proper choice of solder flux. Thermal problems associated with solder fluxes are exacerbated in condensation soldering with its inherently long duty cycle. For example, while materials to be soldered under more conventional soldering techniques such as wave soldering, are exposed to elevated temperatures for only two to three seconds, condensation soldering processes require much longer times, e.g., 25-40 seconds and sometimes even longer. To compensate for such a long duty cycle, a solder flux should be slow acting as compared with a prior art, faster acting water soluble fluxes. Only such slower action could reduce the otherwise severe corrosion problem which may be encountered. Even more importantly, I have discovered that many of the problems with prior art solder fluxes employed in condensation soldering are due to the decomposition of the condensation soldering fluids when in prolonged contact with the flux at elevated temperatures. The soldering fluxes used in condensation soldering gradually accumulate in the sump of the condensation soldering machine. Refluxing of the condensation soldering fluid for an extended period of time in the presence of solder flux chemicals leads to a noticeable decomposition of the condensation soldering fluid. Hence, it is important to obtain a flux formulation which is not only water soluble and slow acting, but that inhibits the decomposition of the condensation soldering fluids. Typical condensation soldering fluids are perfluorinated organic compounds having molecular weights of from 600-700. Examples of such fluids are perfluoroamylamine, perfluorophenanthrene and perfluoropolyethers.